The present invention relates to a process for preparing a lithium ferrite (LiFeO.sub.2) powder having a layered rock salt type structure. Such a lithium ferrite powder is useful as a cathode material for use in secondary lithium batteries, and the like.
Lithium cobalt oxide (LiCoO.sub.2), lithium nickel oxide (LiNiO.sub.2), and solid solutions thereof each having a layered rock salt type (.alpha.-NaFeO.sub.2 type) structure have been studied and developed, and are practically used as cathode materials in secondary lithium batteries now in use as re-chargeable power sources for portable electronic and electric apparatuses. These cathode materials, though high in operating voltage and large in capacity, are however expensive because they contain a rare metal such as Co or Ni. This is therefore a hindrance to extension of the market of secondary lithium batteries wherein they are used (the cost of a cathode material accounts for about 1/3 of the price of a battery).
On the other hand, a lithium manganese spinel such as LiMn.sub.2 O.sub.4 and lithium ferrite (LiFeO.sub.2), which have attracted attention as low-cost 4 V-class cathode materials of the next generation, are under research and development. Lithium ferrite in particular is most expected as a low-cost cathode material of the next generation since it is prepared using Fe as a very rich resource. When this compound is prepared according to a conventional method comprising a solid state reaction of a variety of lithium compound with a trivalent iron compound at 300.degree. to 900.degree. C. e.g., J. C. Anderson and M. Schieber, J. Phys. Chem. Solids, 25 (1964) 961-968!, there is obtained only either .alpha.-LiFeO.sub.2 wherein cations are randomly distributed, or .gamma.-LiFeO.sub.2 having a structure wherein cations are regularly disposed but which is different from that of LiCoO.sub.2. In these phases, no route of diffusion of Li during charging or discharging is secured, whereby the compound fails to exhibit cathode characteristics. Accordingly, establishment of a method of preparing layered rock salt type LiFeO.sub.2 having the same crystal structure as LiNiO.sub.2 or LiCoO.sub.2 has been of urgent necessity. Synthesis of this compound is as of now carried out by subjecting .alpha.-NaFeO.sub.2 synthesized by a conventional solid state reaction method to ion exchange in a molten salt containing Li ions e.g., T. Shirane, R. Kanno, Y. Kawamoto, Y. Takeda, M. Takano, T. Kamiyama and F. Izumi, Solid State Ionics, 79 (1995) 227-233; this will hereinafter be referred to as "Referential Literature"). Since this method requires 2 steps: preparation and ion exchange of .alpha.-NaFeO.sub.2, however, it involves problems such as a difficulty in mass production and a failure in obtaining high-purity crystals. Thus, a novel practical process alternative to this method has been desired.